Conformal coverage of high surface area three-dimensional substrates is typically done by vacuum based techniques like chemical vapor deposition (CVD) and atomic layer deposition (ALD). In spite of their many advantages, these deposition methods also have some drawbacks such as high running and equipment costs, complex set-up, and low throughput due to a tremendous increase in deposition time as the surface area of the substrates increases. For commercial applications there is a strong need to overcome these disadvantages. This could be found in wet chemical routes which are characterized by their operational simplicity and concurrent low cost.
Niesen et al. (J. Electroceram. 6, p.169, 2001) disclose a wet chemical approach allowing deposition of ultrathin TiO2 layers on planar substrates via chemical solution deposition (CSD). This approach is however not particularly suitable for the conformal coating of substrates presenting a relief.
Shaijumon et al. (Adv. Mater. 2010, 22, 4978-4981) disclose the design and making of nanoarchitectured 3D positive electrode for lithium ion microbattery applications. This involved the thermal decomposition of Pechini-like sol-gel LiCoO2 precursors evenly spray-coated onto 3D Al nanorod current collectors directly grown on an Al substrate. For the preparation of the nanostructured LiCoO2, an aqueous solution of Lithium salts, Cobalt salts, ethylene glycol and citric acid was prepared to form a transparent sol and this sol was spray coated onto the Al nanorods, followed by drying in air at 60° C. for 1 hour. The function of ethylene glycol was to form ester cross-linkages. The substrate was rinsed and heated at 650° C. for 8 h in air. The process of spray-coating and annealing was repeated 2 to 5 times to obtain the wanted LiCoO2 coating thickness. The nanorods were however not completely covered. Moreover, the method of Shaijumon et al. involved three spray-coating and annealing steps, totalling a process time of from 27 to 30 hours, resulting in a thickness of only 30 nm.
There is therefore still a need in the art for conformal coating processes at least partly overcoming the above drawbacks.